Technical Manual

Inc.

p 191

APPENDIX 2G

GUIDELINES AND RECOMMENDATIONS

Chapter 2

Helical Foundation Systems

stiff clay soils, although they can be effectively designed for bearing in very dense sands and

hard clay. With proper design and installation techniques, helical piles may also be considered for

bearing on or within soft or weathered bedrock. A competent bearing stratum should be identified

by the geotechnical investigation. Additional helix plates are often needed along the shaft as the

required pile/anchor capacity increases or when bearing in lower-strength soils. Therefore, with

higher pile/anchor capacities, the known or investigated thickness of the competent soil layer must

also increase to fully embed all of the helix plates along the shaft. For compression piles, the

geotechnical investigation should extend at least 5 to 10 feet below the anticipated termination

depth of the pile. Soil strength parameters for the soils within that 5 to 10 feet should be equivalent

to or greater than those strength parameters for the soils at the helix bearing depths.

2. The Standard Penetration Test (SPT) completed in accordance with ASTM D1586 is a common

method of retrieving disturbed soil samples in the field while also providing correlations to several

soil strength parameters. The SPT is performed by driving a 2-inch O.D. split barrel sampler 18

inches with a 140 pound hammer falling a distance of 30 inches. The number of blows required to

drive the sampler the final 12 inches is recorded as the standard penetration number, or N-value.

Typically, N-values of 15 to 30 blows per foot for clay soils and 10 to 25 blows per foot for

sand are preferred for providing the necessary end-bearing resistance for helical piles or helical

anchors. N-values higher or lower than those ranges may also be considered.

3. The standard penetration test provides a reasonable indication of strength and density of granular

soils with correlations available to relate SPT N-values to relative density, unit weight and internal

friction angle. Laboratory direct shear tests or triaxial tests provide even more accurate estimates

of soil strength which may be warranted for large projects. The additional cost of performing these

tests could be offset by a more economical pier design that would not have been considered

using SPT results alone.

4. SPT N-values may be inconsistent for fine-grained, cohesive soils and may not accurately reflect

the soil shear strength. Tests may also be conducted on intact cohesive soil samples with pocket

penetrometers. These results can vary widely between technicians depending upon the accuracy

of the instrument and how closely the test procedure is followed. Laboratory testing of cohesive

samples collected using undisturbed sampling methods, such as Shelby tube sampling (ASTM

D1587), provides more reliable results. The more common methods for laboratory testing of

undisturbed samples of cohesive soils are the unconfined compression test, the triaxial shear

test and the direct shear test. Undrained shear strengths ranging from 1,500 psf to 4,000 psf

are preferred for use of helical piles or anchors, although higher or lower values may also be

considered.

5. The presence of very loose granular soil and very soft to soft cohesive soil should be identified.

Column buckling may be a concern for compression piles when very loose or soft soil is present

in layer thicknesses of more than just a few feet, especially for a deep foundation element that

derives its capacity primarily from end-bearing. Column buckling should be considered when SPT

N-values are less than 4 blows/foot. When SPT sampling indicates weight-of-hammer (WOH) or

weight-of-rod (WOR) values, additional laboratory testing for soil strength is recommended to

document the shear strength in the WOH/WOR zone. Cone penetrometer tests (CPT) completed

in accordance with ASTM D3441 may also be considered to measure in situ soil strengths. The

CPT is widely used in lieu of, or as a supplement to, the SPT. The CPT is particularly suited for

soft clays, soft silts and fine to medium sand deposits.